Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
The fuel cell stack and peripheral equipment (BOP) including components such as a reformer, an evaporator, a heat exchanger, and an exhaust gas combustor are used in combination to form a fuel cell module. In the structure, the reformer reforms a mixed gas, for example, of a raw fuel chiefly containing hydrocarbon and water vapor to produce the fuel gas supplied to the fuel cell stack. Therefore, it is desirable to efficiently mix two fluids, i.e., the raw fuel and the water vapor, that are supplied separately.
For example, in a combustion apparatus for a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2001-201019 (hereinafter referred to as the conventional technique 1), as shown in FIG. 11, a high calorie combustion gas supply pipe 3a is provided for supplying hydrocarbon fuel to a flame hole 2a of a combustor 1a. The high calorie combustion gas supply pipe 3a and a high calorie combustion gas supply pipe 4a for supplying a reactant gas that has been supplied to the fuel cell system, but not consumed in the fuel cell system (off gas), to the combustor 1a jointly form a dual pipe 5a. A plurality of holes 6a are formed at a downstream end of the high calorie combustion gas supply pipe 4a. After the off gas flows through the high calorie combustion gas supply pipe 4a, the off gas flows through the holes 6a and is supplied to the flame hole 2a. 
Further, in a kerosene fuel vaporizing method for a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2004-319330 (hereinafter referred to as the conventional technique 2), as shown in FIG. 12, a vaporizer 1b is used. The vaporizer 1b includes a water vapor inlet pipe 2b and a kerosene oil inlet pipe 3b. An injection port 5b is formed at a front end of the kerosene oil inlet pipe 3b through a tapered portion 4b. A mixing/vaporizing portion 6b is formed on the front side of the injection port 5b. 
The injection port 5b injects the kerosene oil fuel toward the mixing/vaporizing portion 6b, and at the mixing/vaporizing portion 6b, the kerosene oil fuel and the water vapor supplied from the water vapor inlet pipe 2b are mixed with each other to vaporize the kerosene oil fuel.
Further, in an injection device for injecting a mixed fuel disclosed in Japanese Laid-Open Patent Publication No. 2012-057927 (hereinafter referred to as the conventional technique 3), as shown in FIG. 13, a mixed fuel injection nozzle body 1c, a combustion chamber 2c, a spray guidance pipe 3c, and a mixing device 4c are provided. Further, the injection device includes an air pipe 5c, a liquid fuel supply pipe 6c, a water supply pipe 7c, a diffuser 8c, and an injection opening 9c of the spray guidance pipe 3c. 